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KDSS Anti-roll Suspension


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#51 CWA

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Posted 06 March 2012 - 20:17

there are suspension springs , they load up


In the kinetic system, yes. But the SAE paper desmo referred to describes a car with a mode decoupled system (with no warp stiffness) which replaces corner springs entirely.

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#52 DaveW

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Posted 06 March 2012 - 23:39

Interesting read. A few interesting concepts and opinions.

An interesting & seductive read. I confess that I don't know quite how respond, except to say that I was closely involved with a variety of active vehicles from road cars to F1 all of which simulated modal suspensions & I didn't reach the same conclusions. I think that the gist of Mr. Zapletal's argument has merit, but his conclusions fail to recognize the complexity of suspension problem.

Hence, for example, a null warp dynamic stiffness will reduce vehicle response to a warp input, but a vehicle warp displacement will not to be corrected. Also, if a warp load is required to maintain lateral balance in a corner, then there is no way of achieving that without warp stiffness. In truth, a low warp dynamic stiffness will be helpful on some circuits (e.g. Monaco), but would be found to be less acceptable (slow) on more conventional circuits (e.g. Silverstone).

I could continue, but I think might be sufficient to state that F1 suspension set-ups tend to follow some of Mr. Zapletal's reasoning.

Edited by DaveW, 06 March 2012 - 23:41.


#53 gruntguru

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Posted 07 March 2012 - 02:28

Interesting read, that SAE paper. Why not decouple the modes? Do we need warp/twist mode stiffness at all?

Apart from the obvious advantage of even tyre loadings on a "warped" section of road, the lack of warp stiffness results in a reduced stiffness in single wheel bump for a given roll and pitch stiffness.

#54 DaveW

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Posted 07 March 2012 - 09:53

Apart from the obvious advantage of even tyre loadings on a "warped" section of road, the lack of warp stiffness results in a reduced stiffness in single wheel bump for a given roll and pitch stiffness.

Quite right, but that is not the whole story.

The initial input at the front wheels of a kerb strike is not a pure roll input, it is the combination of a roll & a heave input (I neglect warp for now). With a stiff heave spring, a soft roll spring will reduce the load change at the loaded wheel as you suggest, but it will also increase the load reduction at the unloaded wheel. If the input is big enough, then (in this condition) the stiff heave spring will cause the unloaded wheel to become airborne. It is easy to imagine (hopefully), that at some point during the latter stages of a kerb strike both front wheels can become airborne. That is exactly what happened to Kobayashi in Singapore 2011 qualifying when he took too much kerb - his next contact was the outside kerb & his final contact was the wall. A higher roll stiffness might have slowed him down over some parts of the track, but should also have helped him to complete the lap....

Incidentally, with a modally controlled active suspension with self-levelling operating on all modes, it was still necessary to run roll & warp springs to "tidy up" the suspension travel.

Edited by DaveW, 07 March 2012 - 11:04.


#55 gruntguru

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Posted 07 March 2012 - 12:29

The initial input at the front wheels of a kerb strike is not a pure roll input, it is the combination of a roll & a heave input (I neglect warp for now). With a stiff heave spring, a soft roll spring will reduce the load change at the loaded wheel as you suggest, but it will also increase the load reduction at the unloaded wheel.

Am I missing something? Wouldn't low warp stiffness improve the single wheel kerb strike by increasing the load of the 2 nearest wheels? Is this the opposite of what you are saying?

#56 pugfan

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Posted 07 March 2012 - 23:02

...If the input is big enough, then (in this condition) the stiff heave spring will cause the unloaded wheel to become airborne...


I'm a bit confused as well. Considering an axle in isolation for the moment, surely a stiff roll spring and soft heave spring is needed for a lift in one wheel to cause a lift in the other?

#57 jpf

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Posted 09 March 2012 - 19:46

I may be even more confused. How is warp not a linear superposition of pitch and roll?

#58 DaveW

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Posted 09 March 2012 - 23:56

Am I missing something? Wouldn't low warp stiffness improve the single wheel kerb strike by increasing the load of the 2 nearest wheels? Is this the opposite of what you are saying?

I was talking about the initial hit (before the rear wheel encounters the kerb).... Logically, a soft roll/warp stiffness will not move the sprung mass to alleviate the hit the unloaded wheel will take when kerbing with a high heave stiffness. It is difficult to describe, but a modal suspension does always work in a obvious way - I am trying to compile a simple model in an attempt to explain & intend to post when I am happy that it works. Pugfan, please be patient.


I may be even more confused. How is warp not a linear superposition of pitch and roll?

If heave is H*[1,1,1,1], pitch is P*[1,1,-1,-1], roll is R*[1,-1,1,-1] & warp is W*[1,-1,-1,1], then I think you are looking for values for P & R that will equilibrate [W, -W, -W, W] with [P+R, P-R, -P+R, -P-R]. Try it & see....

Edited by DaveW, 10 March 2012 - 00:00.


#59 jpf

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Posted 10 March 2012 - 06:14

Ah, of course. Thanks.

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#60 gruntguru

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Posted 10 March 2012 - 06:20

I may be even more confused. How is warp not a linear superposition of pitch and roll?

Let me re-phrase Dave's answer in layman's terms (the kind I need in order to understand these things). Any combination of pitch and roll will still end up with the four contact patches "in plane" ie a flat road. To warp a suspension requires a warped road or a wheel lift.

#61 DaveW

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Posted 20 March 2012 - 10:51

Am I missing something? Wouldn't low warp stiffness improve the single wheel kerb strike by increasing the load of the 2 nearest wheels? Is this the opposite of what you are saying?

Here is my long-delayed reply.

I modeled the front axle of a vehicle, contacting a notional kerb on its left side. The kerb profile is shown here.

The modal (heave & roll, respectively) responses for a typical F3 setup are shown here, and the contact patch loads (left & right, respectively) are here.

The equivalent modal responses for a vehicle with F1 vertical stiffness levels but no roll stiffness are shown here, and the contact patch loads are here.

To explain, since I modeled the front axle, only the initial responses are relevant, and since the input was vertical, roll stiffness is the same as warp stiffness. For the "F3" case, the strong bar caused a rapid roll response, which backed off the heave response at the unloaded side, reducing the contact patch load variation of that side. Thus, whilst the loaded wheel is launched by the kerb, the unloaded wheel remains in good contact with the road. This contrasts with the the zero roll stiffness case, where the the high vertical displacement is initially "shared" by the unloaded wheel, causing both wheels to lose contact with the road. Hence my comment about Kobayashi's incident & why I don't think that zero warp stiffness is always a good strategy.

Very simplified, but I hope helpful.



#62 gruntguru

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Posted 20 March 2012 - 12:38

Very interesting Dave and thanks for the considerable effort you must have put in. I don't understand your outputs 100%, for example Graphic #1 - is this two bumps/kerbs?

Are you sure your zero front roll stiffness is equivalent to zero warp stiffness (ZWS)? The lack of any roll response in Graphic 3 doesn't fit with my (intuitive only) view of how a ZWS system with normal levels of roll, pitch and heave stiffness would respond. A single wheel kerb strike should produce pitch and roll responses via the lateral and longitudinal "Z bar" or equivalent couplings.

#63 DaveW

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Posted 20 March 2012 - 20:36

Very interesting Dave and thanks for the considerable effort you must have put in. I don't understand your outputs 100%, for example Graphic #1 - is this two bumps/kerbs?

Thanks, but it was not too onerous. I used the opportunity to become more familiar with SciLab. I admitted defeat for a while, becoming convinced that my distant knowledge of Matlab was not helping. Finally I tried SciLab version 5.4 (pre-release) & it came together fairly quickly. My kerb input was obtained by adding together different generator elements, & two peaks was more interesting than one (I thought)...

Are you sure your zero front roll stiffness is equivalent to zero warp stiffness (ZWS)? The lack of any roll response in Graphic 3 doesn't fit with my (intuitive only) view of how a ZWS system with normal levels of roll, pitch and heave stiffness would respond. A single wheel kerb strike should produce pitch and roll responses via the lateral and longitudinal "Z bar" or equivalent couplings.

Perhaps I was being too simplistic, although I have tested an F1 vehicle which had a front roll stiffness (& I too mean roll) rather less than 10 percent of the heave stiffness, courtesy of a stiff "heave" spring & rather notional "corner" springs & bar. Interestingly, the heave stiffness was also around 3 times the tyre stiffness. Such a set-up is extreme and, in the limit, it does require the sprung mass to be moved around to maintain continuous tyre contact.



#64 RDV

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Posted 24 March 2012 - 19:17

GL-Having seen one of these Kinetic systems up close and personal I am not going to write them off.

..one has run the Kinetic system in track racing, and can say very effective, with immense possibilities. Our system had installation problems (never really worked out the bugs in the hydraulics and mechanical bits, our problem, not Kinetics...), more due to a competitive championship...when we got the edge with a conventional car, dropped the Kinetic H2O system as lacked development. Have discussed it with the people who ran it in rally cars, and they were pretty happy with it. Suspect our small suspension movement and mega aero car did not get as much gain from it as a rally car on gravel and flying...

#65 gruntguru

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Posted 27 March 2012 - 04:29

The "Pull to Damp" thread has moved towards a discussion of roll resistance and roll damping which prompted me to post this sketch but I thought it might be better posted here - particularly since the artist is Erik Zapletal mentioned earlier in this thread. I have copied it without permission but I'm sure Erik won't mind.
Posted Image

This suspension layout is a simple implementation of zero warp stiffness with fully decoupled roll and pitch stiffness.

If the longitudinal leaf springs were replaced with rockers, each connected to the chassis by a single coil spring and damper, the roll and pitch damping would also be decoupled and independently adjustable. The warp mode would be undamped. Dive and squat could be controlled seperately without affecting the roll moment distribution which (I think) is entirely kinematic (no elastic component) and a function of side pivot location.

Edited by gruntguru, 27 March 2012 - 04:43.


#66 Johan Lekas

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Posted 27 March 2012 - 08:51

Posted Image
Design with decoupled roll mode, free (undamped) warp mode
Roll resistance distribution front/rear can be changed by adjusting the leverage ratios of the rockers



#67 DaveW

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Posted 27 March 2012 - 19:30

Roll resistance distribution front/rear can be changed by adjusting the leverage ratios of the rockers

Forgive me, Johan, but are you sure. It seems to me that changing roll resistance distribution will, in a turn, introduce a warp displacement of the suspension. Zero warp resistance implies that the displacement will not generate a warp load, so it follows that changing the roll resistance distribution will change the "average" roll resistance, coupled with some warp deflection in a turn, but no change in "balance".



#68 Johan Lekas

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Posted 28 March 2012 - 08:12

... but are you sure.

Well I was, kind of, but now... :)

It seems to me that changing roll resistance distribution will, in a turn, introduce a warp displacement of the suspension. Zero warp resistance implies that the displacement will not generate a warp load, so it follows that changing the roll resistance distribution will change the "average" roll resistance, coupled with some warp deflection in a turn, but no change in "balance".

I think I understand what you mean: If the rockers are the same geometry front and rear the axles can be moved in warp without roll movement of the chassis, and vice-versa. If the geometry of one of the rockers is changed, moving the axles in warp will also give roll movement of the chassis, and vice-versa. I can also see that the average roll resistance changes.
But that it would not change balance (roll resistance distribution front/rear)... I'll have to think about that!
Thanks for your input!


#69 gruntguru

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Posted 07 January 2013 - 00:27

Well I was, kind of, but now... :)

The key here is that regardless of the ERMD setting, the suspension can be "warped" without displacing any of the springs. The fact that the chassis may simultaneously roll a little, depending on the ERMD setting is not relevent.

Johan. Not sure if I asked this question earlier, but what are your thoughts on the complete lack of warp mode damping? I ask because I recently saw University of Western Australia's latest FSAE chassis and it has zero warp stiffness, but with the dampers in the conventional location ie one per wheel.

The UWA layout is functionally similar to the sketch in Post#65 but with the "Longitudinal Leaf Springs" replaced by rocker beams and the pivots attached to an ARB rather than springs.

Edited by gruntguru, 07 January 2013 - 00:31.


#70 Johan Lekas

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Posted 07 January 2013 - 18:44

Johan. Not sure if I asked this question earlier, but what are your thoughts on the complete lack of warp mode damping?

I've been thinking along these lines:
The springs involved when the suspension moves in warp are the tires, and the mass moving consists of the axles and roll suspension components (the chassis does not move). So what could theoretically happen is that there would be some oscillations in this spring/mass system. The frequency would have to be relatively high since the tires are stiff and the mass not that big. The oscillation movement would also have to pass the roll spring/damper unit, which probably could let small displacement movements past without damping. I still think it's unlikely that there will be any oscillations since there is friction in links etc and there is some damping in the tires.

Do you have any pictures of or links to the UWA system? Would be interesting to check it out!


#71 gruntguru

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Posted 07 January 2013 - 23:17

Do you have any pictures of or links to the UWA system? Would be interesting to check it out!

UWA 2012 photos courtesy of Rex Chan - University of Melbourne.

Car is not complete (dampers attached to axles but not chassis etc). Note:
- "E" shaped plate acting as spring & lat/long location link at centre of each (beam) axle.
- Underbody aero (undertray) acts as "twin rocker beams" connecting front and rear axles. Also restrains rotation of axles about Y and Z axes.
- Roll mode controlled by single ARB connected to "rocker beams" at a point (which is longitudinally adjustable to set F/R roll stiffness)
- Final pic shows car with approx 150mm of shim under LF tyre (3 remaining wheels on ground, dampers not connected) to demonstrate soft twist and travel in single wheel bump. These cars are only required to have +- 25mm of travel.

#72 gruntguru

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Posted 09 October 2014 - 23:43

Bump.  John - how is your project car going? There is a new thread here about torsionally soft chassis, which is relevant to your soft-twist suspension setup.

 



#73 imaginesix

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Posted 11 October 2014 - 19:37

Thanks for reviving this thread, I totally missed it the first time around.
 
I'd been tossing around the idea of interconnected suspension on and off since I first read about it in RaceEng magazine in '97. I think there is a very important conclusion that can be made just by considering the consequences on a purely conceptual level.
 
In theory the ideal setup would result in zero motion in roll and pitch modes, unrestricted motion in warp, and +/- stiffness in heave to suit the ride/comfort targets for a given vehicle.
 
Roll - 0
Pitch - 0
Heave - +/-
Warp - infinite
 
The thing is that as any of the modes approach unrestricted motion (such as is supposedly desired in warp), the chassis' attitude easily falls out of working range for anything that we would consider driveable.
 
The most obvious example is heave. With no resistance to the motion of the chassis in heave the car will simply fall onto its bump rubbers, or flat onto the road if there's enough range of motion. Likewise in roll, the car would simply flop over onto the L or R wheel set even at a standstill. Assuming a 1:1 motion ratio between L and R wheel sets, the longitudinal pivot axis for the 'flopping' will be the centerline of wheel tracks. Since the pivot axis would be below the CG, it would be inherently unstable.
 
The same is true for pitch, which would have a transverse pivot axis located at a point proportional to the F/R motion ratios. (I believe it would be essential for the F/R motion ratio to closely mirror the F/R weight bias as a means of setting the LLTD to maintain the handling balance). So for a typical FWD vehicle with 60% front weight bias the pivot point would be 40% of the wheelbase distance behind the front wheels.
Roll-Pitch%2520Axes.png
This all assumes a rigid interconnection in heave. If heave is softened at all, my understanding at this point is that the pivot axes effectively LOWER below ground level, basically aggravating the instability.
 
Now, none of that matters for pitch and roll since the theoretical ideal is a perfectly rigid interconnection in those modes anyways. But where it starts to matter is warp. Can the car collapse with unrestricted motion in warp? It can!
 
For every movement of one diagonally opposed wheel set, the other interconnected wheel set would move in the opposite direction by the same amount. That means the car would have to rise up one way or another if the suspension began to warp. Graphically, the four pivot axes in warp mode (one for each wheel the chassis could collapse onto) would shape a diamond-shaped 'table' under the CG that looks like this for the same vehicle.
Warp%2520Axes.png
 
As a consequence, the car will be stable at rest even with zero warp control. But things start to change when dynamic forces are introduced. The force vectors acting on the CG (probably the wrong terminology) could run over the edges of that 'table' if the CG is high enough and/or the acceleration is strong enough. I visualize it by overlaying a g-g circle over the previous image, which in a real life case would be scaled according to the height of the CG.
Warp%2520Axes%2520with%2520gg%2520circle
Where the g-g circle runs beyond the edges of the pivot axes, the car risks collapsing onto that corner in dynamic situations. In this example, braking while turning would cause one of the front corners to collapse, with the wheel on the opposite corner raising into the air. Things get worse if the surface is off-camber, or if heave mode isn't rigid. Any softness in heave will have the effect of shrinking that diamond platform.
 
Based on this, it's feasible for any car to achieve infinite roll and pitch stiffness (ignoring any other potential detrimental effects). But warp stiffness is necessarily a bit of a balancing act, alongside heave control. In other words, control vs comfort. Not too different from the trade-offs that have to be made when treating suspension tuning in the traditional manner!
 
So while some cars may be able to achieve zero warp resistance safely, most won't. For example that dune buggy in previous posts could never have a soft warp mode without collapsing on every bump, due to the height of its CG. A formula car might pull it off, but I can't begin to hypothesize about the effects of downforce on all this.
 
But if any of this is correct, it demonstrates the value of having 50/50 weight distribution for the sake of being able to benefit from a fuller range of motion at each corner, and for the sake of reducing the compromise between ride and handling. Which all just supports what we already believe; FWD cars are an abomination! :)

Edited by imaginesix, 11 October 2014 - 19:43.


#74 Johan Lekas

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Posted 12 October 2014 - 19:24

Bump.  John - how is your project car going? There is a new thread here about torsionally soft chassis, which is relevant to your soft-twist suspension setup.

It's moving on slowly. Nothing new on the suspension.

The big job left is to finish the bodywork. Then it's electrics and paint.

(Not fully developed aluminum shaping skills set a minimum safe viewing distance of 5 m  ;) )

 

2edzj9k.jpg

 

 I've read the thread on fsae.com on beam axles (http://www.fsae.com/...or-both./page15), and there is some interesting stuff. For example the distinction between warp-soft and twist-soft



#75 gruntguru

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Posted 12 October 2014 - 23:01

Johan that looks awesome. I guess I am more than 5m away.



#76 DaveW

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Posted 15 October 2014 - 09:07

 

 I've read the thread on fsae.com on beam axles (http://www.fsae.com/...or-both./page15), and there is some interesting stuff. For example the distinction between warp-soft and twist-soft

 

When I first encountered an F1 vehicle (many moons ago) two levers were fitted in the cockpit to the right of the driver.  They controlled the stiffness of the front and rear arb's and were used to control the overall roll stiffness and the mechanical lateral balance of the vehicle.  Balance issues were almost never discussed.  They are now banned in all race series, except IRL I think.  Why?

 

On topic, I believe that the longitudinal position of the centre pivot of Zapletal's suspension sets the lateral balance of the vehicle.  If so, then the distinction between "warp-soft" and "twist-soft" is a little blurred, at least in a race vehicle.  Ideally, the position should be adjustable, depending on tyre wear, temperature and pressure (for example).  Alternatively, an additional arb might work.

 

Zapletal almost never mentions the tyres, which is a pity, because the suspension (a conventional one at any rate) is required to damp the unsprung modes.  It is hard to see how that can be achieved without warp damping.



#77 desmo

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Posted 15 October 2014 - 14:35

When I first encountered an F1 vehicle (many moons ago) two levers were fitted in the cockpit to the right of the driver.  They controlled the stiffness of the front and rear arb's and were used to control the overall roll stiffness and the mechanical lateral balance of the vehicle.  Balance issues were almost never discussed.  They are now banned in all race series, except IRL I think.  Why?


Thank you for bringing this up. I've wondered the same for ages and I've never heard anything approaching a remotely logical explanation. Why not let the drivers adjust wing angles and maybe spring preload by corner as well? Are race engineers afraid they'll be put out of a job listening to the drivers incessantly bitching about over/understeer?

#78 Fat Boy

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Posted 15 October 2014 - 16:44

Thank you for bringing this up. I've wondered the same for ages and I've never heard anything approaching a remotely logical explanation. Why not let the drivers adjust wing angles and maybe spring preload by corner as well? Are race engineers afraid they'll be put out of a job listening to the drivers incessantly bitching about over/understeer?

 

The first thing I did to the car I worked on this year was put an adjustable FARB in it. It's a tool, but it's not a panacea. Do you really think that having the ability to change bar settings eliminates the job of an engineer? C'mon.



#79 Fat Boy

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Posted 15 October 2014 - 16:46

When I first encountered an F1 vehicle (many moons ago) two levers were fitted in the cockpit to the right of the driver.  They controlled the stiffness of the front and rear arb's and were used to control the overall roll stiffness and the mechanical lateral balance of the vehicle.  Balance issues were almost never discussed.  They are now banned in all race series, except IRL I think.  Why?

 

Ummm, going to have to check rules packages, Dave. They're very common. The only series I know of that doesn't allow it is World Challenge.



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#80 desmo

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Posted 15 October 2014 - 17:45

The first thing I did to the car I worked on this year was put an adjustable FARB in it. It's a tool, but it's not a panacea. Do you really think that having the ability to change bar settings eliminates the job of an engineer? C'mon.


No, but it might lessen the volume of drivers' complaints.

#81 Fat Boy

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Posted 15 October 2014 - 18:26

It never gets any easier, you just go faster. - Greg Lemond



#82 DaveW

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Posted 15 October 2014 - 22:09

A quote to remember FB....

 

So far as I am aware, no European open wheel series use them (although F1 has its own impressively expensive solution), & I don't recall FAtlantic using them.  Touring Cars used to, but not any more.  I'm not sure about DP.



#83 gruntguru

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Posted 15 October 2014 - 23:00

On topic, I believe that the longitudinal position of the centre pivot of Zapletal's suspension sets the lateral balance of the vehicle.  If so, then the distinction between "warp-soft" and "twist-soft" is a little blurred, at least in a race vehicle.  Ideally, the position should be adjustable, depending on tyre wear, temperature and pressure (for example).  Alternatively, an additional arb might work.

 

Zapletal almost never mentions the tyres, which is a pity, because the suspension (a conventional one at any rate) is required to damp the unsprung modes.  It is hard to see how that can be achieved without warp damping.

I don't think an additional ARB will help - the twist mode and roll moment distribution won't change. (RMD is as you say defined by the position of the centre pivot.

 

Single wheel disturbances will excite roll, pitch and heave modes as well, so single wheel damping will remain significant.

 

Location of the dampers is an interesting point I have wondered about in the past. In Johan's design above, the dampers are located with the springs (at the four pivot points) providing roll, pitch and heave damping but zero warp damping. OTOH the UWA car located the dampers conventionally - one at each wheel.



#84 DaveW

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Posted 16 October 2014 - 08:41

I don't think an additional ARB will help - the twist mode and roll moment distribution won't change. (RMD is as you say defined by the position of the centre pivot.

 

 

Would that be a reason for rejecting the "Balanced Suspension", unless a neat way can be found to control the position of the centre point?



#85 Fat Boy

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Posted 16 October 2014 - 16:59

A quote to remember FB....

 

So far as I am aware, no European open wheel series use them (although F1 has its own impressively expensive solution), & I don't recall FAtlantic using them.  Touring Cars used to, but not any more.  I'm not sure about DP.

 

I really don't know about Euro series, so I can't comment there.

 

All of the older Atlantics had adjustable bars. The last one (Swift 016) only had the front adjustable because it was built to a cost cap and the adjustable rear bar added a couple hundred bucks they didn't want to spend. The teams would have put one one unquestioned, it wasn't allowed.

 

Indycars have them, as do Lights cars (old on the front and new...both). The Pro Mazda and F2000 both adjust the front. The rear would be adjustable, but again, they're not allowed to by the rules. DP cars have both front and rear adjustable. GT cars can/do have adjustable bars in the TUDOR series. They're not allowed in World Challenge...again, a rules thing.

 

I don't believe GT3 cars in Europe run adjustable bars, I don't really know why that is. My experience with the Euro mindset of chassis tuning/development is fundamentally different than the American approach. We spend a lot more time developing what we have in an almost analog progession. They are more likely to race what they have for 'X' amount of time and spend thier development time to introduce the next model as a quantum jump forward. I'm not saying that one is better than the other, but what I'm saying is that an American is more likely to want to have more knobs to spin and the Euro is more likely to leave each model as it is.

 

Again, this isn't some sort of grand law, just tendencies based on my observations.



#86 gruntguru

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Posted 17 October 2014 - 06:27

Would that be a reason for rejecting the "Balanced Suspension", unless a neat way can be found to control the position of the centre point?

It could be viewed as a drawback although I would think the opposite. Shifting the ERMD is as simple as moving a link end forward or backward although that may be tricky to implement in a "driver-adjustable" form. The UWA version had only an ARB connected to the centre point.



#87 Johan Lekas

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Posted 20 October 2014 - 18:27

There are some test results and driver feedback on a comparison between conventional suspension and Creuat's implementation of a "mode separated" suspension system on a GT Viper and an LMP1:

http://www.creuat.co...Viper ORECA.pdf

http://www.creuat.com/rfh.html

 

Interesting comments in the Viper doc regarding the low(er) warp stiffness:

"...b) Lack of steering feedback in fast corners...

...The lack of feedback we believe is due to the change the suspension
makes in the contribution of chassis torsion. Drivers are used to some
chassis deflection that creates a particular feedback over surface
bumps. Chassis is like a spring with no damping, and this has a
particular effect on response to different frequency inputs.
The fact our suspension has a low axle-crossing stiffness eliminates
the role of the chassis torsion. This means that frequency response of
the suspension changes and drivers perceive this as a lack of
feedback.
We are of the thinking that this should be a positive factor and that
drivers could take advantage of it with a proper training."

 

 

Could this be a common characteristic for all twist-soft suspensions?



#88 DaveW

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Posted 21 October 2014 - 07:15

Fascinating...

 

I recall Senna saying something similar about the Lotus active suspension*, on the other hand he did tell a fellow driver that he no longer had to tape up his hands (no power steering)....

 

The quoted settings on the Viper would suggest to me a fairly dramatic turn-in under-steer, washing out as the dampers catch up.  I need to think about the other modes, but I suspect the the roll mode was too stiff (assuming that I have read the units correctly - tyre rates would have been around 600 N/mm).  I wonder if they ran a self-leveling strategy?

 

*  I think the comment went something like "I know how a normal car will behave in a corner without turning the steering wheel, but I have to believe this car will react predictably to a steering input."


Edited by DaveW, 21 October 2014 - 08:01.


#89 DaveW

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Posted 23 October 2014 - 23:15

Assuming that I have interpreted Creuat's nomenclature correctly, their Viper set-up of 30 Sept 2004 was not modally decoupled.  The modal stiffness array was:

 

30    20      0     0

10  170      0     0

  0      0  420    2.5

  0      0    75  27.5

 

Incidentally, their "axle crossing" mode was what I like to call the "warp" mode (rather than Zapletal's "twist" mode).

 

A unit warp displacement (1, -1, -1, 1) mm generated axle loads of (30, -30, -25, 25) Newtons, whilst a unit roll displacement (1, -1, 1, -1) mm generated axle loads of (495, -495, 345, -345) Newtons.



#90 Johan Lekas

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Posted 24 October 2014 - 15:55

I guess the Creuat Viper set-up was not fully mode decoupled because they needed to adjust the elastic roll moment distribution
 

...The first mode configures an isostatic weight distribution. In this mode, the axle crossing is completely free, and the scales would also measure 50%/50% diagonal weights. The second mode includes a slight stiffness of the axle crossing, which is used to change also the front/rear roll stiffness, and therefore the understeer rate...

 
I think with modes decoupled (in heave, pitch, roll, warp) the only way to change ERMD is to move from pure warp (1, -1, -1, 1) to "twist" (ie not pure warp) and thereby couple roll and warp.